TREE_USED and DECL_READ_P

[冠人 王 via gcc]

GCC Edition:7.3.0
I find in source code/gcc/c/c-decl.c , 
Line 1265 to 1281 decides what situation leads to warnings for unused variables
I am confused about line 1266:
I think only the DECL_READ_P is enough for program to 
decides the warning for unused variables, so I can elmiminate
" !TREE_USED(p) || "
Am I right ?
The comment for "DECL_READ_P":
InVAR_DECL and PARM_DECL, set when the decl has been used except for being set